Conventional calibration methods using laser-based trace gas sensors require either separate reference cells or interrupt the measurement to calibrate. In the former, the optical pathlengths of the sample gas and reference cell will have different characteristics including optical fringes, sensitivities, and environmental conditions. All of these factors may complicate the relevance of the reference cell calibration to the sampled gas. The latter technique only provides an intermittent calibration method and thereby stops the measurement intrinsically and requires frequent calibrations under rapidly changing conditions. It would be desirable to provide systems and methods that address these and other shortcomings of existing systems.